The present invention relates to an ionization smoke detector and more particularly, a detector having a housing and sensing chamber which cooperate to control the flow of detected smoke or aerosols having entrained particulate matter.
Ionization smoke detectors are utilized to detect the presence of fires or unsafe smoke conditions and operate by admitting a small portion of air in the monitored environment to a sensing chamber. Convection currents and drafts are generally employed to move smoke-laden air through the sensing chamber. The rate and manner in which air flows through the chamber can have a significant effect upon the sensitivity to smoke at unsafe or hazardous levels. For this reason, a number of detectors as disclosed in U.S. Pat. Nos. 3,731,093, 3,908,957 and 3,935,465 are provided with means for controlling the manner in which ambient air drifts into and through a sensing chamber. In general, special baffling arrangements are provided to control the air and attenuate disturbances caused by wind and strong drafts.
In ionization smoke detectors, winds or drafts have several adverse effects upon sensitivity. First of all, the detector utilizes a radioactive ionizing source that ionizes air between two electrically charged electrodes in the sensing chamber so that a very low ionization current flows even under normal, no-smoke conditions. When unsafe smoke conditions prevail, particulate matter carried by the smoke into the sensing chamber reduces ion flow and the current, and detection circuitry responding to the reduced current is utilized to trigger an alarm signal. Unfortunately, a strong draft or wind through the chamber can remove ions from the chamber faster than they are generated which also reduces the ionization current. Since the detection circuitry is incapable of distinguishing between reduced current caused by smoke and reduced current caused by winds, a false alarm signal can be sounded by the wind in the absence of an unsafe condition.
Accordingly, it is the general object of the present invention to provide an ionization smoke detector which controls air flow through the sensing chamber and maintains sensitivity to smoke conditions in the presence of winds or drafts that exist in smoke-laden air. More specifically, it is an object of the present invention to provide an ionization smoke detector which controls the rate at which air passes through the sensing chamber under all types of wind and draft conditions.